In conventional solid-state ballast for discharge lamps, some form of oscillator or inverter is utilized to drive the load through a power amplifier. It has been found that ambient temperature changes, aging of components, and variations in the impedance of the load produce variations in the drive signal for the load, resulting in variations in the load drive current, and therefore, variations in the illumination provided. Furthermore, it would be desirable to permit variations of illumination under control of an external signal, in order to achieve controlled dimming from a remote location.
Prior to ignition, gas discharge lamps present a substantially higher impedance to the drive circuits than they do during steady-state operation. The removal of one or more lamps from the ballast would similarly result in an increased impedance level. Such high impedance levels could result in the production of dangerously high voltages within the electronic ballast. Such voltages could result in breakdown and destruction of solid-state components, or the electrocution of an individual attempting to change a lamp.
It is an object of the present invention to provide constant current to the load in a solid-state ballast for discharge lamps, independently of ambient temperature variations, component aging, power supply variations, variations in the impedance of the load, and the like.
It is a further object of the present invention to control reliably and conveniently the illumination produced by a solid-state and ballast, from a point remote from the ballast.
It is another object of the present invention to avoid excessively high voltages when a lamp is removed from the ballast, or during start-up, prior to the firing of the lamps.
It is also an object of the present invention to provide a solid-state ballast which is reliable and convenient in use, yet relatively simple and inexpensive in construction.
In accordance with an illustrative embodiment demonstrating objects and features of the present invention, a solid-state ballast for discharge lamps includes a device for sensing the instantaneous current flowing within the load to produce illumination. A signal representing this load current is differentially processed with an external control signal representing the desired illumination, to produce an error signal which controls an oscillator that provides the operating signal for the load. This feedback arrangement results in constant illumination, while also permitting remote dimming control. In addition, the voltage across the load is sensed and the signal representing this voltage is fed back to a circuit which controls the oscillator so as to limit the maximum load voltage during initial start-up and when a lamp is removed.